Turbofan Engine Main Bearing Arrangement

ABSTRACT

A turbofan engine ( 20; 300; 400 ) comprises a fan ( 28 ), a fan drive gear system ( 60 ), a fan shaft ( 120 ) coupling the fan drive gear system to the fan, a low spool, an intermediate spool, and a core spool. The low spool engages at least three main bearings of which at least two are non-thrust bearings and at least one is a thrust bearing. The fan shaft engages at least two bearings ( 148, 150 ). The core spool engages at least two bearings ( 250, 260 ). The intermediate spool engages at least two of said bearings ( 220, 200, 230; 220, 200, 230 - 2; 200, 220, 230 - 3 ).

CROSS-REFERENCE TO RELATED APPLICATION

Benefit is claimed of U.S. Patent Application Ser. No. 61/789,266, filedMar. 15, 2013, and entitled “Turbofan Engine Main Bearing Arrangement”,the disclosure of which is incorporated by reference herein in itsentirety as if set forth at length.

BACKGROUND

The disclosure relates to turbofan engines. More particularly, thedisclosure relates to main bearing arrangements for turbofan engineshaving epicyclic gear reductions.

Gas turbine engines and similar structures feature a number ofsubassemblies mounted for rotation relative to a fixed case structure.Such engines have a number of main bearings reacting radial and/orthrust loads. Examples of such bearings are rolling element bearingssuch as ball bearings and roller bearings. Typically such bearings allreact radial loads. Some such bearings also react axial (thrust) loads(either unidirectionally or bidirectionally). Ball bearings typicallyreact thrust loads bidirectionally. However, if the inner race isconfigured to engage just one longitudinal side of the balls while theouter race engages the other longitudinal side, the ball bearing willreact thrust unidirectionally.

Tapered roller bearings typically react thrust unidirectionally. Twooppositely-directed tapered roller bearings may be paired or “duplexed”to react thrust bidirectionally. An example is found in the fan shaftbearings of U.S. Patent Application Publication 2011/0123326A1. Such fanshaft bearings are widely spaced to behave as two distinct bearingsproviding radial support at two spaced locations so as to adequatelyreact overturning moments or torques normal to the bearing axis (e.g.,pitch and yaw) and thus fully support the fan. Other duplexing examplesinvolve closely spaced bearings which behave as a single bearing andtheir combination may be referred to as a single duplex bearing. Such asingle duplex may need to have a longitudinally spaced apart additionalbearing reacting radial loads (and optionally thrust loads) for theircombination to react yaw and pitch loads.

US Patent Application Publications 2013/0025257A1 and 2013/0025258A1disclose so-called three-spool engines wherein a high pressure spoolcomprises a high pressure compressor (HPC) and a high pressure turbine(HPT) respectively upstream of and downstream of a combustor. Anintermediate spool comprises an intermediate pressure compressor (IPC)upstream of the HPC and an intermediate pressure turbine (IPT)downstream of the HPT. A low spool comprises a low pressure turbine(LPT) downstream of the IPT and driving the fan via a fan drive gearsystem. The exemplary low spool comprises only the LPT and associatedshaft assembly and does not include any compressor stages.

Unless explicitly or implicitly indicated otherwise, the term “bearing”designates an entire bearing system (e.g., inner race, outer race and acircumferential array of rolling elements) rather than the individualrolling elements. The term “main bearing” designates a bearing used in agas turbine engine to support the primary rotating structures within theengine that produce thrust. This is distinguished, for example, from anaccessory bearing (which is a bearing that supports rotating structuresthat do not produce thrust such as the fuel pump or oil pump bearings inan accessory gearbox).

SUMMARY

One aspect of the disclosure involves a turbofan engine comprising: afan; a fan drive gear system; a fan shaft coupling the fan drive gearsystem to the fan. A low spool comprises a low pressure turbine and alow shaft coupling the low pressure turbine to the fan drive gearsystem. An intermediate spool comprises an intermediate pressure turbineand a compressor. A core spool comprises a high pressure turbine and acompressor. The low spool engages at least three main bearings of whichat least two are non-thrust bearings and at least one is a thrustbearing. The fan shaft engages at least two bearings. The core spoolengages at least two bearings. The intermediate spool engages at leasttwo of said bearings.

In one or more embodiments of any of the foregoing embodiments, saidnon-thrust bearings and said thrust bearing are rolling elementbearings.

In one or more embodiments of any of the foregoing embodiments, saidthrust bearing is a non-duplex ball bearing.

In one or more embodiments of any of the foregoing embodiments, saidnon-thrust bearings are roller bearings and said thrust bearing is aball bearing.

In one or more embodiments of any of the foregoing embodiments, theremay be exactly nine said main bearings.

In one or more embodiments of any of the foregoing embodiments, each ofsaid main bearings is either a single stage rolling element bearing or amulti-stage bearing wherein an interstage gap is no more than 30 mm.

In one or more embodiments of any of the foregoing embodiments, the atleast two non-thrust bearings are exactly two.

In one or more embodiments of any of the foregoing embodiments, said onethrust bearing engaging the low spool also engages the intermediateshaft.

In one or more embodiments of any of the foregoing embodiments, two ofthe non-thrust bearings engaging the low spool also engage a case.

In one or more embodiments of any of the foregoing embodiments, of theat least two bearings that engage the fan shaft, at least one is anon-thrust bearing and at least one is a thrust bearing.

In one or more embodiments of any of the foregoing embodiments, of theat least two bearings that engage the core spool, at least one is anon-thrust bearing and at least one is a thrust bearing.

In one or more embodiments of any of the foregoing embodiments, of theat least two bearings that engage the intermediate spool, at least oneis a non-thrust bearing and at least one is a thrust bearing.

In one or more embodiments of any of the foregoing embodiments, of theat least two bearings that engage the fan shaft, at least one is anon-thrust bearing and at least one is a thrust bearing.

In one or more embodiments of any of the foregoing embodiments, of theat least two bearings that engage the core spool, at least one is anon-thrust bearing and at least one is a thrust bearing.

In one or more embodiments of any of the foregoing embodiments, whereinof the at least two bearings that engage the intermediate spool, atleast one is a non-thrust bearing and at least one is a thrust bearing.

In one or more embodiments of any of the foregoing embodiments, one ofsaid at least two non-thrust bearings engaging the low spool engages aturbine exhaust frame.

In one or more embodiments of any of the foregoing embodiments, one ofsaid at least two non-thrust bearings engaging the low spool engages aninter-turbine frame.

In one or more embodiments of any of the foregoing embodiments, the lowspool engages four bearings of which at least one is a non-thrustbearing engaging the low spool and the intermediate spool.

In one or more embodiments of any of the foregoing embodiments, one ofthe bearings engaging the fan shaft is a thrust bearing and one thebearings engaging the fan shaft is a non-thrust bearing.

In one or more embodiments of any of the foregoing embodiments, one ofthe bearings engaging the core spool is a thrust bearing and one thebearings engaging the core spool is a non-thrust bearing.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic longitudinal sectional view of a first turbofanengine.

FIG. 2 is a schematic longitudinal sectional view of a second turbofanengine.

FIG. 3 is a schematic longitudinal sectional view of a third turbofanengine.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a turbofan engine 20 having a central longitudinal axis orcenterline 500. The engine has a structural case including a core case22. The exemplary structural case further comprises a fan case 24connected to the core case by a circumferential array of struts 26 andsurrounding the fan 28. The core case and the fan case may haverespective outboard aerodynamic nacelles (not shown).

The exemplary forward rim of the fan case is proximate an engine inlet30 receiving an inlet flow 502 when the engine is operating. The inletflow passes downstream through the fan 28 and divides into a core flow504 passing inboard along a core flowpath 506 within the core case and abypass flow 508 passing outboard along a bypass flowpath 510 between thecore case 22 and the fan case 24.

The core flow 504 (or a major portion thereof allowing for bleeds, etc.)passes sequentially through one or more compressor sections, acombustor, and one or more turbine sections before exiting a core outlet34. In the exemplary engine the fan is a single-stage fan having asingle stage of fan blades 40. Each of the compressor and turbinesections may include one or more blade stages mounted to rotate as aunit about the centerline 500. The blade stages may be alternatinglyinterspersed with vane stages. Each compressor section is co-spooledwith an associated turbine section. From upstream to downstream alongthe core flowpath, the exemplary engine has two compressor sections 42and 44, the combustor 45, and three turbine sections 46, 48, and 50. Thefan and compressor sections (and their stages) progressively compressinlet air which passes into the combustor for combustion with fuel togenerate high pressure gas which passes downstream through the turbinesections where the gas pressure is progressively reduced as work isextracted. The turbine section 46 operates at highest pressure and isoften referred to as a high pressure turbine (HPT) or a core turbine.The HPT blade stages are connected via a shaft 52 (“high shaft” or “coreshaft”) to the blade stages of the compressor section 44 to drive thatcompressor section (often referred to as a high pressure compressor(HPC) or core compressor) to form a high spool or core spool.

The turbine section 48 operates at an intermediate pressure range and isthus often referred to as an intermediate pressure turbine (IPT). TheIPT blade stages are connected via a shaft 54 (“intermediate shaft”) tothe compressor section 42 to drive that compressor section (oftenreferred to as an intermediate pressure compressor (IPC)) to form anintermediate spool.

The turbine section 50 operates at a low pressure range and is thusoften referred to as a low pressure turbine (LPT). The LPT blade stagesare connected via a shaft 56 (“low shaft”) to a transmission 60 (e.g.,an epicyclic transmission, more particularly a geared system known as afandrive gear system (FDGS)) to indirectly drive the fan 28 with a speedreduction.

An exemplary high pressure turbine 46 is a single or double stageturbine assembly; an exemplary intermediate stage turbine 48 is a singleor double stage turbine assembly; an exemplary low pressure turbine 50is a multi-stage turbine (e.g., three or more).

The exemplary transmission 60 comprises a central externally-toothed sungear 80. The sun gear 80 is encircled by an internally-toothed ring gear82. A number of externally-toothed star or planet gears 84 arepositioned between and enmeshed with the sun gear 80 and ring gear 82.The star or planet gears 84 can be referred to as intermediate gears. Acage or carrier assembly 86 carries the intermediate gears viaassociated bearings 88 for rotation about respective axes. The bearings88 may be rolling element bearings or may be journal bearings (e.g.,having external circumferential surface portions closely accommodatedwithin internal bore surfaces of the associated intermediate gears 84).

The exemplary carrier assembly 86 comprises a front plate (e.g.,annular) in front of the gears and a rear plate (e.g., annular) behindthe gears. These plates may be mechanically connected by the bearings 88and/or by linking portions between adjacent intermediate gears.

In the exemplary embodiment, a forward end of the low shaft 56 iscoupled to the sun gear 80. The exemplary low shaft 56 has a generallyrigid main portion 100 and a flexible forward portion 102. A forward endof the portion 102 may have a splined outer diameter (OD) surfaceinterfitting with a splined inner diameter (ID) surface of the sun gear80 to transmit rotation.

The exemplary carrier assembly 86 is substantially non-rotatably mountedrelative to the engine case 22. In the exemplary embodiment, the carrierassembly 86 is coupled to the case 22 via a compliant flexure 110 thatallows at least small temporary radial and axial excursions androtational excursions transverse to the centerline 500.

The exemplary ring 82 is coupled to the fan 28 to rotate with the fan 28as a unit. In the exemplary embodiment a rear hub 122 of a main fanshaft 120 connects the fan 28 to the ring gear 82.

The speed reduction ratio is determined by the ratio of diameters of thering gear 82 to the sun gear 80. This ratio will substantially determinethe maximum number of intermediate gears 84 in a given ring. The actualnumber of intermediate gears 84 will be determined by stability andstress/load sharing considerations. An exemplary reduction is betweenabout 2:1 and about 13:1. Although only one intermediate gear 84 isnecessary, in exemplary embodiments, the number of intermediate gears 84may be between about three and about eleven. An exemplary gear layoutwith fixed carrier is found in U.S. Patent Application Publication2012/0251306A1.

Thus, the exemplary engine 20 has four main rotating components (units)rotating about the centerline 500: the core spool (including the highpressure turbine 46, the high shaft 52, and the high pressure compressor44); the intermediate spool (including the intermediate pressure turbine48, the intermediate shaft 54, and the intermediate pressure compressor42); the low spool (including the low pressure turbine 50 and low shaft56); and the fan assembly (including the fan 28 itself and the fan shaft120). Each of these four things needs to be supported against: radialmovement; overturning rotations transverse to the centerline 500; andthrust loads (parallel to the axis 500). Radial and overturningmovements are prevented by providing at least two main bearings engagingeach of the four units. As is discussed below, such at least two aresufficiently axially spaced to resist the overturning movements.

Each unit would have to also engage at least one thrust bearing. Thenature of thrust loads applied to each unit will differ. Accordingly,the properties of required thrust bearings may differ. For example, thefan 28 primarily experiences forward thrust and, therefore, the thrustbearings engaging the fan 28 may be configured to address forward thrustbut need not necessarily address rearward thrusts of similar magnitudes,durations, etc.

The FIG. 1 embodiment has placed two main bearings 148, 150 along thefan shaft forward of the transmission 60. Inboard, the inner race ofeach bearing 148, 150 engages a forward portion of the shaft 120 aft ofthe fan 48. Outboard, the outer race of each bearing 148, 150 engagesstatic structure of the case. The exemplary static structure comprises asupport 152 extending inward and forward from a forward frame 154. Thesetwo bearings 148, 150 thus prevent radial excursions and overturningmoments which the fan 48 may produce during flight.

To resist thrust loads, one or both of the bearings 148, 150 may bethrust bearings. In an exemplary embodiment, the bearing 150 is anon-thrust bearing (e.g., straight roller bearing with longitudinalroller axes configured to only handle radial loads). The other bearing(i.e., the bearing 148) is a thrust bearing. Due to the significance offorward thrust loads on the fan 28, the bearing 148 may be biased toresist forward loads. The exemplary bearing 148 is a bidirectional ballbearing or a bidirectional tapered roller bearing (e.g., wherein therollers have a forward taper and forwardly converging roller axes topreferentially handle the forward thrust loads). A similar bidirectionaltapered roller bearing is shown in U.S. Pat. No. 6,464,401 of Allardentitled “High Load Capacity Bi-Directional Tapered Roller Bearing”.Ball bearings are typically bidirectional thrust bearings. However, aunidirectional ball bearing may be formed by having at least one of theraces contacting only a single longitudinal side of the balls.

An exemplary bearing arrangement for supporting the remaining threeunits is discussed below. Various aspects of each of these may beindependently implemented or all may be implemented in a given engine.

The low shaft 56 is principally radially supported by a forward bearing170 and an aft bearing 172. The exemplary forward bearing 170 may bedirectly radially grounded to the case 22 (e.g., via a bearing support174 extending inward from the frame 154 aft of the support 152).

The exemplary bearing 172 is radially grounded to the case 22 via asupport 180 extending inward from a frame 182 extending across the coreflowpath 504. The exemplary support 180 is aft of the LPT 50 with theframe 182 being a turbine exhaust frame. Alternative implementations mayshift the support 180 forward of the LPT 50 to engage an inter-turbineframe between the turbine sections 48 and 50.

In the exemplary embodiment, both of the bearings 170 and 172 arenon-thrust bearings, such as non-thrust roller bearings (e.g., is astraight roller bearing). An additional inter-shaft thrust bearing 200(e.g., a bidirectional ball bearing) has an inner race engaging anintermediate portion of the low shaft 56 and an outer race engaging theintermediate shaft 54 to indirectly axially ground the low shaft 56 tothe case 22 via the intermediate shaft 54.

The intermediate spool is supported by forward bearing 220 and an aftbearing 230. In an exemplary embodiment, the forward bearing 220 is abidirectional thrust bearing directly radially and axiallysupporting/grounding the intermediate spool via a support 240 extendingto the front frame 154 (e.g., just aft of the support 174). Alternativeembodiments might ground to an inter-compressor frame 242 between thecompressor sections 42 and 44. An exemplary bearing 220 is a singlestage bidirectional ball bearing.

A single bidirectional duplex bearing (e.g., two oppositely configuredunidirectional ball or roller thrust stages) may also be used as thebearing 220 or other thrust bearing. The close positioning of the twostages may be needed to avoid problems associated with differentialthermal expansion of the two bodies (spools or static structure betweenwhich the bearings radially intervene). With large gap between stages(e.g., measured as the longitudinal span between the ends of the rollingelements of the first stage and the adjacent ends of the rollingelements of the second stage) differential thermal expansion couldeither cause bearing disengagement or excessive thrust loads. A smallgap (e.g., no more than the individual axial spans of the rollingelements of one or both stages, more broadly no more than 1.5 timestwice such axial span) will avoid such problems. In an exemplary gasturbine engine, such a gap may be not more than 30 mm or not more than25 mm. For example, the intermediate spool and high spool may be subjectto greater heating than the case and thus greater thermal expansion. Ifone of these is supported relative to the case by two widely spacedthrust stages, differential thermal expansion may be a problem. Incontrast, the fan shaft and the adjacent portion of the low spool may beat relatively uniform temperature and thus the two bearings 148 and 150may be more widely spaced (shown not to scale so that the relative gapappears smaller than it would be in an actual engine).

The bearing 230 (e.g., a non-thrust bearing, such as a non-thrust rollerbearing) indirectly radially supports/grounds the intermediate spool byengaging the intermediate spool and the low spool or, namely, engagingthe intermediate shaft 54 and the low shaft 56 with an outer raceengaging the intermediate shaft 54 (e.g., aft of the turbine 28 and aninner race engaging the low shaft 56 (e.g., just ahead of the turbine50).

The radial loads on the intermediate spool at the bearing 230 willprimarily be transmitted to the low shaft 56 and through an aft portionof the low shaft 56 to the bearing 172 and grounded by the support 180and frame 182. Axial (thrust) loads will pass through the bearing 220.

Thus, thrust loads on the low spool are transmitted via the shaft 56through the bearing 200, through the portion of the intermediate shaftthereahead, to the bearing 220, and grounded back through the support240.

The core spool may be fully directly supported by two bearings 250 and260 of which at least one would be a thrust bearing. In the exemplaryembodiment, the bearing 250 is a forward bearing grounding a forwardportion of the core shaft ahead of the compressor section 44 to theinter-compressor frame 242 via a support 270. The aft bearing 260grounds a portion of the core shaft aft of the turbine section 46 via asupport 272 to an inter-turbine frame 274 between the sections 46 and48. In alternative embodiments, this aft bearing 260 might be shiftedintermediate the compressor section 44 and turbine section 46 and thesupport 272 may extend to a combustor frame (not shown). In theexemplary implementation, the bearing 250 is a thrust bearing (e.g., abidirectional ball bearing with its inner race engaging the core shaft52 and its outer race engaging the support 270). The exemplary bearing260 is a non-thrust bearing such as a straight roller bearing with itsinner race engaging the core shaft 52 and its outer race engaging thesupport 272.

FIG. 1 further shows the FDGS 60 as having a centerplane 516 and thegears as having a gear width W_(G) and the fan blade array as having acenterplane 518. From fore to aft, the bearings have respectivecenterplanes 520, 522, 524, 526, 528, 530, 532, 534, and 536.

The FIG. 2 engine 300 is otherwise similar to engine 20, with twochanges. First, intershaft non-thrust roller bearing 230 is replaced bya non-thrust roller bearing 230-2 grounding the intermediate shaft 54directly to the case 22. In this example it grounds a portion of theshaft 54 aft of the turbine 48 to an inter-turbine frame 232 via support231. Alternatives might involve grounding to inter-turbine frame 274(e.g., FIG. 3 below). Second, non-thrust roller bearing 172 is replacedby a non-thrust roller bearing 172-2 ahead of the turbine 50 groundingthe low shaft 56 directly to the case 22 via the frame 232 and aforwardly-shifted support 180-2.

The FIG. 3 engine 400 is otherwise similar to engine 20, with onechange. The intershaft non-thrust roller bearing 230 is replaced by anon-thrust roller bearing 230-3 grounding the intermediate shaft 54directly to the case 22. In this example it grounds a portion of theintermediate shaft 54 forward of the turbine 48 to inter-turbine frame274 via support 231-3.

The use of “first”, “second”, and the like in the following claims isfor differentiation within the claim only and does not necessarilyindicate relative or absolute importance or temporal order. Similarly,the identification in a claim of one element as “first” (or the like)does not preclude such “first” element from identifying an element thatis referred to as “second” (or the like) in another claim or in thedescription.

Where a measure is given in English units followed by a parentheticalcontaining SI or other units, the parenthetical's units are a conversionand should not imply a degree of precision not found in the Englishunits.

One or more embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, whenapplied to an existing basic engine configuration, details of suchconfiguration or its associated environment may influence details ofparticular implementations. Accordingly, other embodiments are withinthe scope of the following claims.

What is claimed is:
 1. A turbofan engine (20; 300; 400) comprising: afan (28); a fan drive gear system (60); a fan shaft (120) coupling thefan drive gear system to the fan; a low spool comprising: a low pressureturbine (50); and a low shaft (56) coupling the low pressure turbine tothe fan drive gear system; an intermediate spool comprising: anintermediate pressure turbine (48); and a compressor (42); a core spoolcomprising: a high pressure turbine (46); and a compressor (44); and aplurality of main bearings, wherein: the low spool engages at leastthree of said main bearings, of which at least two are non-thrustbearings (170, 230, 172; 170, 230-2, 172-3; 170, 230-3, 172) and asingle one is a thrust bearing (200); the fan shaft engages at least twoof said main bearings (148, 150); the core spool engages at least two ofsaid main bearings (250, 260); and the intermediate spool engages atleast two of said main bearings (220, 200, 230; 220, 200, 230-2; 200,220, 230-3).
 2. The engine of claim 1, wherein: said thrust bearing is anon-duplex ball bearing.
 3. The engine of claim 1, wherein: saidnon-thrust bearings and said thrust bearing are rolling elementbearings.
 4. The engine of claim 3, wherein: said non-thrust bearingsare roller bearings; and said thrust bearing is a ball bearing.
 5. Theengine of claim 1, wherein: there are exactly nine said main bearings.6. The engine of claim 5, wherein: each of said nine said main bearingsis either a single stage rolling element bearing or a multi-stagebearing wherein an interstage gap is no more than 30 mm.
 7. The engineof claim 5, wherein: the at least two non-thrust bearings engaging thelow spool are exactly two.
 8. The engine of claim 7, wherein: said onethrust bearing engaging the low spool also engages the intermediateshaft.
 9. The engine of claim 7, wherein: said two non-thrust bearingsengaging the low spool also engage a case (22).
 10. The engine of claim5, wherein: of the at least two bearings that engage the fan shaft, atleast one is a non-thrust bearing and at least one is a thrust bearing.11. The engine of claim 5, wherein: of the at least two bearings thatengage the core spool, at least one is a non-thrust bearing and at leastone is a thrust bearing.
 12. The engine of claim 5, wherein: of the atleast two bearings that engage the intermediate spool, at least one is anon-thrust bearing and at least one is a thrust bearing.
 13. The engineof claim 1, wherein: of the at least two bearings that engage the fanshaft, at least one is a non-thrust bearing and at least one is a thrustbearing.
 14. The engine of claim 13, wherein: of the at least twobearings that engage the core spool, at least one is a non-thrustbearing and at least one is a thrust bearing.
 15. The engine of claim13, wherein: of the at least two bearings that engage the intermediatespool, at least one is a non-thrust bearing and at least one is a thrustbearing.
 16. The engine of claim 1 wherein: one of said at least twonon-thrust bearings engaging the low spool engages a turbine exhaustframe.
 17. The engine of claim 1 wherein: one of said at least twonon-thrust bearings engaging the low spool engages an inter-turbineframe.
 18. The engine of claim 1 wherein: the low spool engages fourbearings (170, 200, 230, 172) of which at least one is a non-thrustbearing (230) engaging the low spool and the intermediate spool.
 19. Theengine of claim 1, wherein: one of the bearings engaging the fan shaftis a thrust bearing (148) and one the bearings engaging the fan shaft isa non-thrust bearing (150).
 20. The engine of claim 1, wherein: one ofthe bearings engaging the core spool is a thrust bearing (250) and onethe bearings engaging the core spool is a non-thrust bearing (260).